A wireless ad hoc network refers to an autonomous system of fixed or mobile nodes connected by wireless links, the union of which form an arbitrary network topology in which nodes can serve as relay points for communication between other nodes. A mobile ad hoc network refers to a wireless ad hoc network in which the nodes are free to move randomly and to organize themselves arbitrarily. Ad hoc wireless networks continue to rise is popularity because they requires minimal configuration, are quick to deploy, and can be used in situations where fixed communication infrastructure does not exist or has been destroyed.
Nodes in a wireless ad hoc network often communicate using a carrier sense multiple access (CSMA) communication protocol under the IEEE 802.11 standard. Nodes in a CSMA protocol contend for access to a shared transmission medium or channel. A node wishing to transmit a packet (the “sender node”) first listens to (or “carrier-senses”) the shared medium for a defined duration to determine if there are any transmissions on the medium, and transmits only if the medium is sensed idle. Carrier sensing is usually determined by antenna sensitivity and controlled by thresholds applied to the level of (actual or apparent) perceived signal strength. In order to combat interference from hidden nodes (i.e., nodes that are capable of interfering with reception of a packet at a receiving node while not necessarily being able to receive the transmitted packet), some CSMA protocols supplement physical sensing with a reservation scheme (also known as virtual sensing). In the reservation-based CSMA, nodes exchange short request to send (RTS) and clear to send (CTS) packets with neighboring nodes prior to the actual transmission. Both packets specify the length of time needed to transmit the actual data packet. Any third party node receiving any of the two packets uses this information to predict when the medium will be available for a future transmission, and must refrain from initiating any communication until the medium is free. On the receiving side, the collision avoidance model of CSMA requires that a packet be received in the absence of any interference for the reception to be deemed successful. The complete exchange is a four-way handshake involving four packets—RTS, CTS, DATA, and ACK—with the final ACK providing notification of successful transmissions from the target node back to the sender node.
The resource allocation and collision avoidance models of the conventional CSMA protocol, however, are overly pessimistic. Specifically, these models of conventional CSMA fail to exploit the capability of the underlying physical layer module of the network to successfully decode multiple packets received in overlapping transmissions, referred to herein as multi-user detection (MUD) or multi-packet reception (MPR). These pessimistic models result in fewer nodes being able to transmit at any given time due to unnecessary backoffs resulting from sensing another carrier on the medium. On the reception side, receiving nodes may unnecessarily abort transmissions due to perceived interference, even when the physical layer module has successfully decoded the received packets.